DESCRIPTION: Large improvements in performing even the simplest perceptual tasks as a result of practice or training have been observed in adult humans in virtually every sensory modality. Identifying the mechanisms by which the adult brain achieves perceptual learning is an important question that will improve our understanding of neural development and may also allow the creation of adaptive systems that mimic human behavior. The investigators' research applies a powerful method for identifying and characterizing the mechanisms of perceptual learning in visual tasks due to fine-tuning of the perceptual template (external noise exclusion), stimulus enhancement, or internal noise suppression. The method adds systematically increasing amounts of external noise - random visual noise (similar to random TV noise) - to the visual stimulus and observes the effect on a perceptual task as perceptual learning takes place. The effects on task performance of adding external noise of the appropriate character (white Gaussian random noise) can be modeled quantitatively. The three mechanisms of perceptual learning yield three "signature" patterns of performance. (E.g., fine-tuning of the perceptual template only affects performance at high levels of external noise because the external noise is large enough to be the limiting factor in performance and it can be better eliminated by a better-tuned perceptual template.) The investigators develop and apply this method to characterize the mechanisms of perceptual learning in a wide variety of perceptual tasks including discrimination and identification or classification of both simple and complex visual patterns. Combining this method with transfer manipulations for stimulus feature, eye of origin, and scale invariance will provide information about the level of learning. The work improves the empirical and theoretical understanding of the nature of perceptual learning. Characterization of perceptual learning mechanisms is necessary to a full understanding of the adaptive nature of the adult human brain. The investigators' theory and methods provide a basis for developing adaptive models of the human brain, evaluate performance in high noise environments, and may contribute to the development of efficient training procedures in applied settings.